The invention relates to a surge protector designed to provide surge protection for broadband coaxial systems and which incorporates both a surge protection device and a fail-short mechanism.
Various types of surge protectors are used to protect electrical equipment from electrical power surges induced by lightning for example. When connected between an electrical conductor and ground, a conventional surge protector conducts electrical current only when a power surge having a voltage in excess of a predetermined voltage occurs on the conductor, in which case the power surge is transmitted through the surge protector from the conductor to ground.
Some surge protectors are also provided with a fail-short mechanism, which is a device that protects against longer-duration power surges. When connected between an electrical conductor and ground, a fail-short mechanism conducts electrical current only in response to a power surge of a relatively long duration. Once the fail-short mechanism becomes conductive in response to a power surge, it remains conductive at all times thereafter (unless it fails due to inability to carry the fail-short current).
A number of surge protectors which incorporate fail-short mechanisms are disclosed in the prior art. For example, U.S. Pat. No. 5,224,012 to Smith discloses a surge protector for use in telephone central offices having a fail-short mechanism which includes a conductive canister 70, a fusible pellet 72, and a spring 90 which biases the canister 70 downwards. The Smith fail-short mechanism has two operating positions, a first position in which the bottom portion of the canister 70 is spaced from a conductive plate 42, as shown in FIG. 3 of the Smith patent, and a second position in which the canister 70 makes contact with the plate 42, as shown in FIG. 4 of the Smith patent. The Smith fail-short mechanism moves from the first position to the second position when the fusible pellet 72 melts due to a prolonged power surge.
If it were to be used to protect a broadband coaxial system in which signals up to one gigahertz were transmitted, the Smith surge protector described above would adversely affect the frequency response of the system due to the relatively large capacitance between its components, including the capacitance between the canister 70 and the conductive plate 42 as shown in FIG. 3 of the Smith patent. That relatively large capacitance, which fail-short mechanisms typically possess, would prevent higher-frequency signals from being transmitted through the surge protector with acceptable insertion and return losses.
Surge protectors which incorporate fail-short mechanisms must also have a minimum current-carrying capability. Such current-carrying capability is typically defined with respect to a number of minimum current levels and the durations which the fail-short mechanism must carry each of those current levels without failure. For example, standards promulgated by Underwriters Laboratories and Bell Communications Research require that a fail-short mechanism be able to handle the following current levels for at least the following durations: 30 Arms for 15 minutes, 60 Arms for 3 seconds, 120 Arms for 0.6 seconds, and 350 Arms for 40 milliseconds.
A fail-short mechanism having a large current-carrying capacity generally requires a larger structure. However, that larger structure is likely to have a relatively large capacitance, which would limit the use of such a device to lower-frequency systems.